In the most common positive displacement hydraulic machines the fluid chambers undergo cyclical variations in volume following a roughly sinusoidal function. It is known from EP0361927 that a chamber can be left to idle by holding an electromagnetically actuated valve, located between the working chamber and the low-pressure source, in the open condition. Thus the output of such synthetically commutated machines is varied through the action of first filling each working chamber with liquid, then deciding whether to reject the liquid back to the low-pressure source or to pump it at pressure to the output manifold. Pumping the liquid back to the low-pressure source means that a very small amount of power needs to be expended, during the time that a working chamber is idle, whilst still allowing the working chambers to become productive with a minimum latency period.
U.S. Pat. No. 4,496,847 showed a first fluid working machine (a pump) absorbing power from a renewable energy device such as a wind or tidal turbine, and a second fluid working machine driving a generator to create electricity. However, a problem is found in maintenance of the renewable energy device when it is required to rotate the shaft of the pump, since the torque required may be very high. This need may be felt during maintenance or starting of the renewable energy device for example.
The fluid working machine mentioned in the first paragraph has desirable properties of efficiency and controllability, but it cannot absorb energy from a fluid source and apply it to its shaft to rotate that shaft. EP0494236 introduced an additional operating mode and controllable valve which allows the use of the machine in a motoring cycle where torque is applied to the rotating shaft, thus allowing a controllable bi-directional energy flow. WO/2008/029073 introduced yet another operating mode and an improved valve which allows the hydraulic machine to start from stationary, which was not possible for the machine of EP0494236. However, the valve required for this machine is complicated and therefore expensive, and does not provide optimal flow for fluid working machines which are operated almost entirely as pumps.
From the above it will be appreciated that there are some applications in which it may be desirable to rotate the shaft of a fluid working machine, but where there is insufficient external torque available to do this and the fluid working machine is not capable of rotating its own shaft. It is, therefore, an object of the present invention to provide an apparatus for and method of rotating the shaft of a hydraulic pump.